Enable basic GitHub Actions CI

.github/workflows/ci.yml

@@ -0,0 +1,12 @@
+name: ci
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-java@v1
+        with:
+          java-version: 1.8
+      - uses: gradle/wrapper-validation-action@v1
+      - run: ./gradlew build

README.md

@@ -1,3 +1,7 @@
+# librootjava
+
+[![ci][1]][2]
+
 Example code for "How-To SU"
 
 For some outdated background details, see:
@@ -418,3 +422,6 @@ dependencies {
     implementation 'eu.chainfire:libsuperuser:1.0.0.+'
 }
 ```
+
+[1]: https://github.com/Chainfire/libsuperuser/workflows/ci/badge.svg
+[2]: https://github.com/Chainfire/libsuperuser/actions